The process of dynamically optimising signal and protocol parameters for a given instantaneous channel condition is known as link adaptation. With the example of an enhanced Node B (eNb) base station communicating with a user equipment (UE), the link adaptation process involves the UE sending feedback information to the eNb with details of the channel conditions. By doing so, the eNb can adjust its transmission parameters to achieve the greatest throughput without surpassing a predetermined error threshold. The effectiveness of the link adaptation process is dependent on both the accuracy of this feedback information and how rapidly it can be calculated and sent. If the eNb is provided with frequent and accurate feedback information it will be able to pick the best possible transmission parameters for that moment and rapidly alter them as required.
CSI may be used as the feedback information in the link adaptation process. CSI includes three separate indicators that are particularly useful when the wireless system is capable of utilising multiple-input multiple-output (MIMO) systems, such as those used in the Long Term Evolution (LTE) standard (or other modern wireless communication standards like HSPA+, 4G, WiMax and IEEE 802.11n). These three indicators found in CSI are:                1. Rank Indicator (RI). The RI reflects the maximum number of spatially parallel transmissions supported by the channel that can be efficiently exploited by the detector or receiver. It informs the eNb whether a higher throughput link (higher rank) should be used over a more reliable link (lower rank). The RI is an indicator of rank.        2. Precoding Matrix Indicator (PMI). The PMI signals the codebook index of the precoding matrix that should be applied at the eNb. The PMI is an indicator of precoding.        3. Channel Quality Indicator (CQI). Formerly referred to as modulation and coding scheme (MCS), the CQI indicates the recommended MCS selection from the given modulation (such as QPSK, 16 QAM and 64 QAM) and code rate (e.g. 1/10, ⅓, ½, ¾, 9/10 etc.) alphabet. The CQI is an indicator of channel quality.        